Chapter 14: The Nervous System

Introduction

The diagnostic examination of the nervous system requires testing of its specific functional components many of which are evaluated while taking the history and examining the body by regions. When evidence of nervous system malfunction is encountered, a complete, systematic neurologic examination is required. The first objective of this systematic examination is to identify all cognitive, sensory, motor, and coordination deficits. From this inventory, the site(s) and mechanism(s) of injury can by hypothesized using the following general principles:

1. Deficits of intellect, memory, or higher brain function imply lesions of the cerebral hemispheres.

2. Deficits of consciousness indicate lesions of the brainstem reticular activating system or bilateral cerebral damage.

3. Paralysis with loss of deep tendon reflexes indicates a lower motor neuron (LMN) lesion interrupting the reflex arc. This can be at the spinal cord, spinal root, plexus or peripheral nerve level. Acute upper motor neuron (UMN) lesions can be associated with decreased reflexes initially, but produce increased reflexes after hours to days.

4. Paralysis with an accentuated deep tendon reflexes (spasticity) indicates an UMN lesion. This may reflect disease of the hemisphere, brainstem, or spinal cord.

5. Unilateral loss of touch and position sensation and contralateral loss of temperature and pain sensation indicate a unilateral lesion of the spinal cord ipsilateral to the loss of touch and position. This happens because the ascending tracts for touch and position sensation decussate in the medulla, whereas the ascending tracts for pain and temperature sensation cross near where they enter the spinal cord.

6. Paralysis is contralateral to lesions above the medulla and ipsilateral below. This is because the descending motor tracts, like the tracts for discriminative sense, decussate in the medulla.

7. An LMN paralysis accompanied by anesthesia in an appropriate distribution usually indicates a peripheral nerve lesion, because many nerves carry both motor and sensory fibers. Sometimes spinal root or segmental cord lesions cause similar signs.

8. Muscle wasting with fasciculation results from an LMN lesion; without fasciculation, wasting is often attributable to intrinsic muscle disease.

Overview of the Nervous System

Interpretation of the neurologic examination requires a comprehensive understanding of the anatomy and functional organization of the nervous system, which is beyond the scope of this book. The reader should consult anatomy and neurology texts for detailed discussions of neuroanatomy and functional physiology.

Anatomic Organization of the Nervous System

For diagnostic purposes, the nervous system is divided anatomically into the brain, the spinal cord, and the peripheral nerves.

Superficial anatomy of the nervous system

The brain is encased within the rigid skull and the spinal cord within the spinal canal of the vertebral column. The ganglia, plexuses, and nerve trunks are inaccessible to physical examination lying deep to the muscles and bones of the spine, chest, abdomen, and pelvis. The peripheral nerves, however, are distributed in the neurovascular bundles, along with ...